Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in
devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttlingmotion of the ring-shaped component, cyclobis(paraquat-
p-phenylene) (CBPQT
4+) (
denoted as the ring),between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along adumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is themost stable and thus
designated the ground-state co-conformer (GSCC), whereas the other co-conformationwith the ring surrounding the DNP ring system is less favored and so
designated the metastable-stateco-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs)of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic moleculardynamics (MD) studies with a force field optimized from DFT calculations and we report several experimentsthat validate the predictions. On the basis of both the total energy per rotaxane and the calculated stressthat is parallel to the surface, we find that the optimal packing
density of the SAM corresponds to a surfacecoverage of 115 Å
2/molecule (one molecule per 4 × 4 grid of surface Au atoms) for both the GSCC andMSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing
density.We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing
density(65 Å
2/molecule, the 3 × 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tiltof the ring with respect to the normal is
= 39
deg.gif"> and 61
deg.gif">, respectively, while the tilt angle of the entirerotaxane is
= 41
deg.gif"> and 46
deg.gif">, respectively. Although the tilt angle of the ring
decreases with
decreasingsurface coverage, the tilt angle of the rotaxane has a maximum at 144 Å
2/molecule (the 4 × 5 grid/molecule)of 50
deg.gif"> and 51
deg.gif"> for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF
6-) staylocalized around the ring during the 2 ns MD simulation. On the basis of the calculated
density profile, wefind that the thickness of the SAM is 40.5 Å at the optimum coverage for the GSCC and 40.0 Å for MSCC,and that the thicknesses become less with
decreasing surface coverage. The calculated surface tensionat the optimal packing
density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This differencesuggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verifiedby experiments on Langmuir-Blodgett monolayers of amphiphilic [2]rotaxanes.
